Contactless reader devices as defined in the opening paragraph are broadly used nowadays, in particular in the service sector, in the field of logistics, in the field of commerce and in the field of industrial production. Examples, which are more based on a smart card, are access systems for buildings and offices, payment systems as well as smart cards for identifying individuals (e.g. passport). Examples, which are more based on the so-called RFID tags, are systems for identification/pricing of objects and products, animal tagging, and paper with an RFID tag inside. It will of course be appreciated that the examples above just show a small segment of the numerous applications and are just for illustrating what smart cards and RFID tags are used for. In addition, the upcoming technology of Near Field Communication, NFC for short, also applies to the same technical field. In the following, RFID tags and smart cards, as well as NFC devices, which are in the passive mode, are generally referred to as transponders.
For all these systems, reader devices are needed (not mandatory for NFC as the devices may also act as readers), which reader devices are to communicate with transponders within the radio range. Therefore, a reader device sends out radio signals, which can comprise commands or data and which can be received by a transponder. In addition, data and commands can be sent back to the reader by the transponder. Transponders include passive and active transponders. In the case of a passive transponder, the transponder also uses the energy of the electromagnetic field to power itself, whereas active transponders have their own power source, e.g. a battery. Furthermore, different kinds of coupling between a reader device and a transponder can be distinguished, such as inductive coupling, electromagnetic backscatter coupling, close coupling and electrical coupling. Finally, the various systems operate in different frequency ranges, starting from some kHz up to several GHz. For the sake of brevity, in the following reference is only made to general system characteristics and—where appropriate—to special system characteristics, e.g. to inductively coupled systems or to the concept of load modulation in inductively coupled systems. However, one will easily perceive that the object of the invention and the measures taken to achieve this object are applicable to all kinds of identification systems.
One problem which is to be overcome when designing a reader device relates to the question how to handle different boundary conditions of the systems, that is to say: varying distances between reader device and transponder, varying environmental conditions (disturbing objects in the radio range), and last but not least different design guidelines, which could mean that the developer has to use a special electric reader circuit with a special antenna.
There are some examples in the prior art, which show different approaches. One of them is disclosed in WO 99/43096, “Data communications terminal and method of adjusting a power signal generated therefrom”, dated Aug. 26, 1999.. Here, a data communications terminal includes an antenna circuit for delivering a power signal to a portable data device. The terminal further encompasses a method of automatically adjusting the power seen by the portable data device without any communications feedback from the portable data device by monitoring an impedance characteristic for the antenna circuit. When a change in the monitored impedance characteristics is detected, the data communication terminal adjusts the power level for the power signal delivered to the portable data device.
WO 99/43096. discloses that the controlled variable is the current through the antenna of a reading device or the antenna's impedance. The plant (controlled system) includes the output amplifiers and the set value is the supply voltage of the output amplifiers. With this system, varying distances between reading device and transponder can be compensated, thus avoiding damage to a transponder through heating it up when the distance gets smaller, and avoiding loss of transmitted data when the distance gets larger. However, WO 99/43096. fails to deal with the input stage of a reading device. Hence, it is an object of the invention to avoid damage to the input stage of a reading device and in particular to provide optimum operation of said input stage.